Fresnel lens for projection screens

ABSTRACT

The present invention is a screen, such as is used in back-lit projection screens, having a Fresnel lens laminated to another layer for support. The screen includes a Fresnel lens having an output surface, and a dispersing screen supportingly attached on a first side to the output surface of the Fresnel lens.

RELATED APPLICATIONS

This is a continuation of parent application Ser. No. 09/229,198, filedon Jan. 13, 1999, now issued as U.S. Pat. No. 6,407,859.

BACKGROUND

The present invention is directed generally to a Fresnel lens for usewith projection screens, and particularly to a Fresnel lens that reducesthe effect of ghost images.

Fresnel lenses are often used in projection screens for collimatinglight received from the illumination source. The Fresnel lens istypically used to increase the gain of the screen at the screen edge, sothat a viewer does not notice a lack of brightness uniformity across thescreen.

However, a Fresnel lens typically generates a ghost image, which is theresult of internal reflections within the lens and the substrate towhich the lens may be attached. The ghost image may be perceived by theviewer, with the effect that the image quality is reduced and the viewermay be distracted. Consequently, the screen manufacturer has tocompromise between brightness uniformity and image quality.

Therefore, there is a need for a Fresnel collimating lens, for use witha projection screen, that reduces, or avoids, the production of ghostimages. The Fresnel collimating lens should also maintain the capabilityof effectively collimating light to provide more uniform brightnessacross the screen.

SUMMARY OF THE INVENTION

Generally, the present invention relates to a screen having a Fresnellens laminated to another layer for support.

In one embodiment of the invention, a screen includes a Fresnel lenshaving an output surface, and a dispersing screen supportingly attachedon a first side to the output surface of the Fresnel lens.

In another embodiment of the invention, a screen includes a Fresnel lenshaving an output surface, where at least a portion of the output surfaceincludes a Fresnel structure. A first optical layer has a first surfacesupportingly attached to the output Fresnel structured surface of theFresnel lens.

In another embodiment of the invention, a first layer has a firstsurface, and a redirecting means for redirecting light passing throughthe screen, has a Fresnel structured output surface. Attaching means onat least one of the first layer and the redirecting means supportinglyattaches the output surface of the redirecting means to the firstsurface of the first layer.

In another embodiment of the invention, a layer of transparent materialhas an input surface and a Fresnel-structured output surface havingridges formed between functional slopes and riser slopes, at least someof the ridges being truncated with flat portions essentially parallel tothe input surface.

In another embodiment of the invention, a layer of transparent materialhas an output surface with a Fresnel-structured portion proximate anedge thereof and a substantially unstructured center portion.

In another embodiment of the invention, a first layer has a firstsurface, and a Fresnel lens having a Fresnel-structured output surface.The Fresnel-structured output surface includes functional slopes andriser slopes, and at least a portion of one functional slope and aportion of a riser slope are embedded in the first surface of the firstlayer.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 illustrates the illumination of a screen without the use ofcollimating optics;

FIG. 2 is a graph showing gain at the center and edge of the screen ofFIG. 1 as a function of viewing angle;

FIG. 3 illustrates the illumination of a screen using a Fresnel lens forcollimation;

FIG. 4 illustrates the creation of ghost images in a Fresnel lens;

FIGS. 5A-5F illustrate different embodiments of embedded Fresnel lensesaccording to the present invention;

FIG. 6 illustrates the occurrence of an inactive region in a Fresnellens;

FIGS. 7A-7C illustrate flat-top Fresnel lenses according to embodimentsof the present invention;

FIG. 8 is a graph showing gain at the center and edge of a screen as afunction of viewing angle for diffusing screens with and without aFresnel lens.

FIG. 9 illustrates a Fresnel pattern over an entire screen area;

FIG. 10 illustrates a partial Fresnel pattern over a portion of ascreen, according to the present invention;

FIGS. 11A, 11B and 12 illustrate cross-sections through differentembodiments of partial Fresnel screens according to the presentinvention;

FIG. 13 illustrates a viewing apparatus according to the presentinvention;

FIGS. 14A and 14B illustrate applications using screens with Fresnellenses having nonuniform focal profiles;

FIGS. 15A-15D illustrate different embodiments of Fresnel lenses havinggrooves to avoid air entrapment during lamination and lensmanufacturing; and

FIG. 16 illustrates a first-surface Fresnel lens according to anembodiment of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to Fresnel lenses, and is believedto be particularly suited to Fresnel lenses for use with rear projectionscreens and monitors One of the advantages of the invention is that theappearance of ghost images is reduced, if not prevented altogether.Therefore, the invention may be used to improve the uniformity of thebrightness perceived across the screen while retaining the quality ofthe image.

Among the many factors important in the design of rear-projectionscreens and monitors are i) efficient light use, ii) a high resolutionand iii) a small form factor. A high efficiency is desirable so that thepower of the light source may be reduced, thus reducing problems withdisposal of waste heat, and reducing energy costs. There is a trendtowards increasingly higher resolution, for example in high definitiontelevision (HDTV), to provide the viewer with a sharper, clearer image.Also, it is generally desired to reduce the form factor, such as volume,footprint or weight, so that the monitor takes up as little space in theuser's environment as possible. The implementation of a large screensize, under the restriction of a small form factor leads to the use ofwide-angle optical systems. Wide angle optical geometries place higherrequirements on the optical components of the screen than exist withnarrow angle systems. The present invention addresses this need forwide-angle components, while permitting efficient light use and highresolution operation. The present invention also permits the maintenanceand/or improvement in screen resolution, brightness and brightnessuniformity from screen center to screen edge.

Consider a rear projection optical system 100 as shown in FIG. 1, havinga light source 102 that illuminates a rear projection screen 104. Thelight source 102 is located at a distance d from the screen 104 and thehalf-angle cone of light emitted by the source 102 is given by θ. Thedistance from the center of the screen 104 to the edge is given by t. Aviewer's eye is typically centrally located at position 106 at adistance r from the screen 104. The angle formed between a normal to thescreen 104 at the screen edge and the viewer's eye is given by the angleα.

At the edge of the screen 104, the light is incident on the rear surface108 at an angle equal to θ, where θ is measured relative to a normal tothe screen. Therefore, the forward direction of light traveling throughthe screen 104 continues through the screen at an angle θ. The viewerperceives only that portion of light from the edge of the screen 104that has been scattered through an angle equal to θ+α. Accordingly, theviewer perceives that the brightness at the edge of the screen isreduced.

This is illustrated further in FIG. 2 which shows the measured values ofgain for a screen with a viewing angle of 64°. In other words, acollimated beam of light incident on the back surface of the screen isscattered into a cone having an angle of 64° as measured by the pointswhere the intensity falls to half of the maximum. The measured gaincurves are for illuminating the screen at angles of θ=0° (continuousline, 202), and θ=2020 (dashed line, 204). The viewing angle, α, is theangle of the ray of light relative to the normal to the screen.

At the central position 106, the viewer views the center of the screen104 at a viewing angle of zero, which is normal incidence. At the sameposition 106, the viewer views the edge of the screen at an angle α.Consider first the case where the light incident on the screen 104 isnot redirected between the light source and the screen, for example by acollimating Fresnel lens. In this case, the light detected by the viewerfrom the center of the screen 104 was incident on the screen at an angleθ=0°, and so we use the upper curve 202. Since the viewer is lookingdirectly at the center position of the screen, the viewing angle α=0°.Therefore, the gain for light at the center of the screen is 0.97, pointA.

The light detected by the viewer from the edge of the screen wasincident on the screen at an angle θ=20°, and so we use the lower curve204. The viewer sees light from the edge of the screen at a viewingangle of α=−30°, i.e. light that has been scattered through an angle of50° (20°+30°). Therefore, the gain for light at the edge of the screen104 is 0.51, point B. Therefore, the screen brightness perceived by theviewer is 47% less at the screen edge (gain=0.51) than at the screencenter (gain=0.97). This large drop in brightness across the screen 104is undesirable and may be very noticeable to a viewer.

In one approach to substantially increase the uniformity of brightnessperceived across a screen, a Fresnel lens 302 may be used to redirectthe light from the light source 102 prior to incidence on the rearsurface 108 of the screen 104, as illustrated in FIG. 3. In theillustrated case, the light incident on the screen 104 is collimated.The light propagates through the edge of the screen 104 in a directionparallel to the light passing through the center of the screen 104.Therefore, the light perceived by the viewer at position 106 from theedge of the screen does not need to be scattered through an angle ofθ+α, but only an angle α. Since, in this case, all light is incident onthe screen 104 at an angle of θ=0°, we use only the upper curve 202.Again, the light reaching the viewer from the center of the screen(α=0°) has a gain of 0.97. However, in this case, the gain of the lightreaching the viewer from the edge of the screen, α=30°, is 0.78, pointC.

Therefore, the perceived drop in brightness from the center to the edgeof the screen 104 is only about 20% when the light reaching the screen104 is collimated by the Fresnel tens. This figure may be furtherimproved if the Fresnel lens is configured to bend the light at the edgeof the screen towards the viewer.

The use of a Fresnel lens, however, introduces additional difficulties.For example, the Fresnel lens is normally supported either around itsedge or on its input surface, since there is typically an air gapbetween the Fresnel lens output surface and the following opticalcomponent. Where the Fresnel lens is edge-mounted, the lens is maderelatively thick to so that there is some degree of self-support,otherwise the lens may move or droop into contact with other components,and change the optical characteristics of the system. Where the Fresnellens is supported on its input surface, the lens is typically attachedto a transparent sheet, such as a sheet of glass. In both of theseapproaches, the Fresnel lens, or Fresnel lens/support combination, isrelatively thick.

A Fresnel lens typically includes at least one structured surface, eachportion of the structured surface lens having a functional surface thatis angled with respect to the lens in order to re-direct light passingthrough that particular portion. Adjacent functional surfaces aretypically connected by riser slopes. The functional surfaces and riserslopes typically present a grooved structure when viewed incross-section. The pattern of functional slopes and riser slopes isgenerally referred to herein as a Fresnel structure.

One problem associated with Fresnel lenses is the generation of ghostimages, which is discussed with reference in FIG. 4. An incoming ray 402enters a Fresnel lens 400 as internal ray 406 and then exits the lens400 on an angled face 404, also referred to as a functional slope. Theray 418 that propagates through the functional slope 404 is redirectedto form the primary image. However, a portion of the internal ray 406 isreflected by the functional slope 404 as reflected ray 408. A portion ofthe reflected ray 408 may be reflected off the entrance face 410 of theFresnel lens is reflected ray 412. The reflected ray 412 is incidentupon another facet such as a riser slope 414 of the lens 400 and passesout as emerging ray 416. The emerging ray 416 forms a secondary image.The ghost image formed by the emerging ray 416 may be perceived by theviewer and detracts from the quality of the primary image presented tothe viewer, and effectively reduces the resolution of the imagepresented to the viewer. Therefore, it is desirable to reduce the effectof the ghost image.

There are two major approaches to reducing the effects of ghosting. Oneis to reduce the spatial separation between the primary and the ghostimages in the plane of the screen to the point where there is nodetectable separation, and the other is to reduce the amount of light inthe ghost image.

The displacement of the ghost image relative to the primary image isdependent on a) the thickness of the Fresnel lens and b) the distancebetween the Fresnel lens and the screen and c) the angle of thefunctional surface. Reducing a) the Fresnel thickness and b) theseparation between the screen and the Fresnel, or both, results in areduction in the displacement of the ghost relative to the primaryimage. Also, reducing the angle of the functional surface results in areduction of the separation between the ghost image primary image. Theangle of the functional surface depends in part on the distance from thecenter of the lens and the difference in refractive index between theFresnel lens material and the material into which the primary ray 418travels.

When the spatial separation between the ghost and primary images in theplane of the screen is reduced to the point where the ghost imageilluminates the same pixel as the primary image, then the viewer isunable to detect a ghost image. In such a case, the resolution of theimage on the screen is unaffected by the Fresnel lens, while theadvantages of increased brightness uniformity and light use efficiencyare maintained. For example, in the case of high definition television(HDTV) having 1024 pixels across the screen, if the ghost image isseparated from the primary image by less than approximately 0.098% ofthe screen width, then no transverse ghost image is visible.

In some situations, the Fresnel lens may be supported on a transparentsheet, such as a sheet of glass, that is attached to the input face ofthe Fresnel lens. Where this is the case, the reflected ray 412 thatleads to the production of the ghost image may predominantly arise fromreflection off a face of the transparent sheet, rather than the inputsurface of the Fresnel lens: the reflection at the interface between theFresnel lens and the transparent sheet may be small due to indexmatching. In such a case, the separation between the ghost and primaryimages is not only dependent on the thickness of the Fresnel lens, butalso the thickness of the transparent sheet.

Earlier approaches to reducing the brightness of the ghost image includedepositing an absorptive coating on the riser slopes 414 of the Fresnelstructure. This is difficult to do without the absorptive coatingspreading on to an adjacent functional face. Another approach is to makethe riser slopes 414 highly scattering, so that the light 416 exitingthrough the riser slopes 414 is highly scattered, thus reducing thebrightness of the ghost image. Again, it is difficult to make the riserslopes 414 highly scattering without adversely affecting the functionalsurface 404. In addition, this does not eliminate the ghost image 416,but scatters it, resulting in a reduction in resolution.

The input face 410 of the Fresnel lens 400 may be treated, for examplewith a matte finish, to reduce the amount of light specularly reflectedinto the reflected ray 412. This approach may, however, reduce theamount of light entering the Fresnel lens, or may scatter light, thusaffecting resolution. The input face 410 and/or the functional face 404may also be treated with an anti-reflection coating to reduce the amountof light reflected into ray 412. This approach has limited utility,however, since anti-reflection coatings have a limited bandwidth andeffective cone angle, outside of which the reflection is notsignificantly reduced. Therefore, since the anti-reflection coating onthe input face 410 and/or functional surface 404 has to operate over awide range of wavelengths and incident angles, the anti-reflectioncoating is not a very satisfactory approach to reducing the brightnessof the ghost.

It is important to note that a mild matte finish or an antireflectioncoating may also be provided on the input face 410 of the Fresnel lens400 in order to reduce ghost images that arise from specular reflectionsfrom the input face 410 interacting with the optical system used withthe screen, for example turning mirrors.

Another approach to reducing the brightness of the ghost image is toprovide some optical interaction within Fresnel lens itself, by loadingthe Fresnel lens with optically interacting particles 422, to reduce theghost image. Examination of FIG. 4 shows that light in the primary image418 travels only a short path through the Fresnel lens 400, whereas thelight 416 in the ghost image travels a much longer distance within theFresnel lens. If the optically interacting particles 422 disposed withinthe lens 400 are scattering particles, then the light in the reflectedbeams 408 and 412 has a high probability of being scattered, for exampleas scattered ray 420, before emerging as the ghost image 418. On theother hand, the light 418 in the primary image has a smaller probabilityof being scattered. Furthermore, since the ghost image 416 has arelatively long path length within the lens 400, there is typically agreater separation between the ghost image and the light scattered fromreflected beams 408 and 412. Therefore, the ghost image 416 may be madeless noticeable because of significant scattering, while there is only asmall reduction in resolution in the primary image 418. The degree ofscattering within the Fresnel lens is selected to reduce the ghost imagewhile maintaining the primary image.

In another approach, the optically interacting particles 422 disposedwithin the Fresnel lens 400 may be absorbing particles, in which casethe light 416 in the ghost image has a high probability of beingabsorbed due to its long path length within the Fresnel lens 400. On theother hand, the light 418 in the primary image has a smaller probabilityof being absorbed. Therefore, the ghost image 416 may be made lessnoticeable because a significant fraction of its light has beenabsorbed, with only a small reduction in brightness of the primary image418, and no affect on the resolution. The degree of absorption withinthe Fresnel lens 400 is selected to reduce the ghost image 416 whilemaintaining the primary image 418. It will be appreciated that theoptically interacting particles 422 may include a mixture of absorbingand scattering particles.

In one embodiment of the present invention, the thickness of the Fresnellens is reduced, with a resultant reduction in the separation betweenthe ghost image and the primary image. In the present invention, thethin Fresnel lens may be supported by being attached to an optical layeron the output side of the Fresnel lens. Supporting the Fresnel lens onthe optical layer on the output side of the lens also reduces theseparation between the lens and the optical layer, further reducing theseparation between the ghost and primary images.

One particular approach to supporting a thin Fresnel lens is illustratedin FIGS. 5A-5C, which show different embodiments of an “embedded”Fresnel lens, where at least a portion of the Fresnel lens is embeddedin the screen.

Considering first the embodiment illustrated in FIG. 5A, the screen 500is formed from a support layer 504 and the Fresnel lens 502. The Fresnelstructure 506 of the Fresnel lens 502 is embedded completely in thesupport layer 504. The support layer 504 may be a diffusing screen film.However, there is no requirement that the support layer 504 be adiffusing film, and there is no intention to limit the invention tosuch. The support layer may be another suitable type of dispersingscreen, such as a lenticular screen, a beaded screen, a surfacediffusing screen, a holographic diffusing, or a micro-structureddiffusing screen. This list is not intended to be exhaustive.

An example of a support layer including a beaded screen, for example asis described in U.S. patent application Ser. No. 09/192,118, andincorporated herein by reference, is illustrated in FIG. 5D. The screen560 includes a Fresnel lens 562 embedded in a first transparent layer564 having a lower index of refraction than the Fresnel lens 562. Alayer of refracting beads 566 is disposed between the first transparentlayer 564 and a second transparent layer 568. The beads 566 are embeddedin a layer of absorbing material 570 that prevents light from passingthrough the interstices between the beads. The upper surfaces of thebeads 566 receive light 572 from the Fresnel lens 562. The light 572 isfocused by the beads 566, with the result that the light 572 divergesafter passing through the screen 560.

An example of a screen 580 having a surface diffuser screen isillustrated in FIG. 5E. The screen 580 includes a Fresnel lens 582embedded in a first transparent layer 584 having a lower index ofrefraction than the Fresnel lens 582. The transparent layer 584 islaminated to a surface diffuser 586 having a refractive index differentfrom that of the transparent layer 584. Light passes through the Fresnellens 582, where it is re-directed, through the first transparent layer584 and then through the surface diffuser 586: the light is typicallyscattered when passing through the diffusing surface 588 of the surfacediffuser 586. In the example illustrated, the transparent layer islaminated to the diffusing surface 588 of the surface diffuser 586. Thediffusing surface 588 may be, for example, a holographic diffusingsurface as shown, or may be a random or microstructured surface. Thediffusing surface 588 may also be on the output surface of the surfacediffuser 586, rather than the input surface. Furthermore, the surfacediffuser may also be loaded with scattering particles to provideadditional bulk diffusion.

The redirecting effect of a functional surface of the Fresnel structuredepends on the difference in refractive index between the material ofthe Fresnel lens and the material into which the redirected rayspropagate. In a conventional Fresnel lens with the Fresnel structure inair, there is a large refractive index difference because the redirectedrays pass into air from the Fresnel lens. In the case of an embeddedFresnel, the difference in refractive index is reduced, since thesupport layer 504 has a refractive index higher than that of air. It isgenerally advantageous to increase the difference between refractiveindices of the Fresnel lens 502 and the support layer 504. Materialsthat may be used for the Fresnel lens 502 include polycarbonate,polystyrene, epoxy acrylates and modified acrylates, or other suitablematerials, such as a resin loaded with fine, high index inorganicparticles. Materials that may be used for the support layer 504 includefluoropolymers and acrylics, such as polyvinyl fluoride, celluloseacetate, cellulose tri-acetate or cellulose acetate butyrate. The designof the Fresnel structure 506 is based on the refractive index differencebetween the lens 502 and the support layer 504, so that light rays 508and 510 entering the Fresnel lens 502 from an illumination sourcepositioned at a design distance from the Fresnel lens 502 source, emergefrom the Fresnel lens 502 into the support layer 504 in paralleldirections.

The Fresnel lens need only redirect light towards the viewer to have abeneficial effect, and need not collimate the light. Nevertheless, incertain situations, collimation may be preferred in order to maximizeoverall screen performance. Therefore, in the description of theinvention, the use of the term redirecting should be understood toinclude redirecting light through the Fresnel lens so as to be moreadvantageous to the viewer. This covers redirecting light so that thebeam of light diverges from the Fresnel lens; parallelizing, orcollimating, the light so that the transmitted beam essentially neitherdiverges not converges, and redirecting the light beam so as to convergeat some point beyond the Fresnel lens. This range of possibilities maybe regarded as bending the light so that it emerges from the Fresnellens at one, or more, angles selected within a continuum of anglesranging from very little redirection, in which case the light divergesfrom the lens, to a significant amount of redirection, in which case thelight converged from the lens.

It should also be appreciated that the focal length of the Fresnelstructure need not be constant across the width of the lens. Forexample, the focal length of the Fresnel lens reduce from a high valueat the center of the lens to a low value the edge of the lens. In such aconfiguration, the light in the center portion may be barely affected bythe lens, while the light at the edge of the lens is redirected througha large angle. Moreover, the profile of the focal length, i.e. the valueof focal length compared with position across the lens, need not besymmetrical, but may be asymmetrical so as to direct light towards oneedge of the screen. This may be useful where, for example, the screen1400 is mounted close to a wall 1402 and the viewer 1404 is positionedaway from the wall 1402, as shown in FIG. 14A. This may also be usefulwhere the screen 1410 is positioned at the edge of an array 1412 ofscreens, and it is desired to direct the light towards the viewer 1414who is positioned centrally relative to the array 1412 of screens.

Several different methods may be used for making the screen 500. TheFresnel lens 502 may be formed using one or more of several differentmethods, including, but not limited to, embossing, extrusion, castingand curing, compressive molding and injection molding. After the Fresnellens 502 has been formed, the support layer 504 may be formed by one ofa number of coating techniques. For example, polymeric material for thesupport layer 504 may be poured on to the Fresnel structure, and thematerial knife-coated thereover to fill in the grooves of the Fresnelstructure. The polymeric material may then be processed, for examplecured, dried, or cooled, to create a permanent support layer 504.Without limiting the invention, the polymeric material may be UVcurable, solvent-based, solventless, dryable, or thermoplastic. Othercoating techniques that may be used include rolling, dipping, diecoating, spinning, and spray coating.

It will be appreciated that a complementary process may be followed,where the support layer 504 is formed first, having the complement ofthe Fresnel structure on one surface. In such a case, the support layer504 may be formed by a process such as embossing, extrusion, casting andcuring, compressive molding and injection molding. The Fresnel layer maythen be formed on top of the support layer 504 using a coatingtechnique, for example as described in the previous paragraph. Thecomplementary surface on the support layer 504 acts as a mold to formthe Fresnel lens.

In another approach to an embedded Fresnel screen, illustrated in FIG.5B, the screen 520 includes a Fresnel lens 522 attached to a centrallayer 526. The support layer 524 is attached to the other side of thecentral layer 526. In this embodiment, the central layer 526 is formedfrom a material having a lower refractive index than that of the Fresnellens 522. The central layer may be, for example an adhesive layer, suchas a pressure sensitive adhesive, iso-octal acrylate-acrylic acidcopolymer or thermoplastic hot melt adhesive. The three layer structuremay be assembled by lamination, thermoforming, compression molding, orultrasonic or RF welding.

It will be appreciated that the design of the Fresnel lens structure 506and 522 in the respective embodiments takes into account the change inrefractive index for light propagating from the Fresnel lens 502 or 522into the adjacent layer 504 and 526, respectively. Therefore, the designof the Fresnel lens need not be identical to a design for a Fresnel lensoperating in air. However, the Fresnel groove structure 506 and 528 isdesigned to substantially redirect light propagating through the Fresnellens 502 and 522 in a preferred direction, which may includecollimation.

In another approach to an embedded Fresnel, illustrated in FIG. 5C, thescreen 540 includes a Fresnel lens 542 attached to a central layer 546.The support layer 544 is attached to the other side of the central layer546. The Fresnel structure 548 of the Fresnel lens is partially embeddedin the central layer 546, leaving air gaps 550 between the functionalslopes 552 of the Fresnel structure and the central layer 546. Such ascreen may be formed, for example, by coating the support layer 544 witha thin layer of adhesive to form the central layer 546, and thenpressing the Fresnel lens 542 through the adhesive central layer 546.The screen 540 may then be processed to fix the central layer 546, forexample by UV curing, heating, cooling, drying and the like. Anadvantage of this embodiment is that a thin Fresnel lens is providedwith support, while still maintaining a large refractive differencebetween the lens material and air. The support layer 544 may be adiffusing layer.

Another approach to an embedded Fresnel is illustrated in FIG. 5F. Ascreen 1560 includes a Fresnel lens 1562 attached to a central layer1566. The support layer 1564 is attached to the other side of thecentral layer 1566. The output surface 1568 of the Fresnel lens 1562 hasa Fresnel structure 1570 that includes rising slopes 1572 and functionalslopes 1574. The peak 1576 between adjacent functional and rising slopes1572 and 1574 of the Fresnel structure 1570 need not have the simpletriangular cross-section as illustrated in FIG. 5C, where part of thefunctional slope is embedded in the central layer 1546. In this case,the peak 1576 has an embedded portion 1578 having differentcross-sectional shape. The case illustrated has embedded portions thatare square or rectangular in cross-section. Other cross-sections may beused. An advantage provided by the embedded portion 1578 is that thepeak 1576 may suffer less damage when being pushed through the centrallayer 1546, than the structure illustrated in FIG. 5C.

The embedded portions 1578 may correspond to the inactive, or unused,portions of the Fresnel structure 1570, which are larger closer to theedge of the lens 1562. In one approach, the embedded portions 1578 maybe larger closer to the edge than the center of the lens 1562, withsmaller embedded portions 1580 closer to the center of the lens 1562.Furthermore, the center portion of the lens 1562 may be provided withthe embedded portions 1580, even though corresponding functional slopeshave no inactive portions. The embedded portions 1580 at the center ofthe lens 1562 need not significantly affect the re-directingcapabilities of the lens 1562, since light propagating through thecenter portion of the lens 1562 typically requires less re-directionthan light propagating through the edge of the lens 1562.

It will be appreciated that the partially embedded Fresnel lenses 542and 1562 illustrated in FIGS. 5C and 5F may be also be formed fromFresnel lenses partially embedded directly into the respective supportlayers 544 and 1564, rather than being partially embedded into a centrallayer.

The embedded Fresnel approach permits the Fresnel lens to be very thin,for example below 0.010 inches thick, thus substantially reducing theseparation between the primary image and the ghost image formed by theFresnel lens.

Another approach for mounting a Fresnel lens to a support layer isdescribed with reference to FIGS. 6 and 7. This approach is termed the“flat-top” Fresnel. The basis of the flat-top Fresnel approach isdescribed with reference to FIG. 6, which illustrates a portion of aFresnel lens 600 and two rays 602 and 604 incident on the input face 606of the lens 600. Each ray 602 and 604 is refracted upon entering thelens 600 to produce internal rays 602A and 604A, respectively. Theinternal rays 602A and 604A are incident on the functional surface face608, and are refracted upon passing therethrough, to produce redirectedrays 602B and 604B.

A portion 608A of the functional surface 608 remains optically unusedsince it lies in the shadow of the adjacent riser slope 610.Accordingly, that triangular portion of material labeled ABCincorporating the inactive portion 608A is not used to redirect lightpassing through the Fresnel lens 600. This triangular portion ofmaterial ABC may be removed from the Fresnel lens 600 to produce a flatsurface 612 along the line AB. The surface 612 may be used for attachingthe Fresnel lens 600 to a supporting film.

A screen 700 incorporating a flat-top Fresnel lens is illustrated inFIG. 7A. The screen 700 is formed from a Fresnel lens 702 that iscontacted to a support layer 704. The support layer 704 may be, forexample, a diffusing screen. The Fresnel lens 702 has a Fresnelstructure 706 with truncated tips 708 and 709 that have respective flatsurfaces 710 and 712 for attaching to the support layer 704. The widthof the flat surface 712 of the outer tips 708 is typically wider thanthe width of the flat area 710 of the inner tips 708, because the angleat which light is incident on the Fresnel lens is greater towards thelens edge, thus creating a larger “shadow” region at the edge that maybe removed to produce the flat contacting surfaces.

The flat-top Fresnel screen 700 is advantageous in that it provides asubstantial flat area for attaching the Fresnel lens 702 to the supportlayer 704, thus providing support to a thin Fresnel lens. The flat-topdesign also maintains an air gap between the active portions of theFresnel structure 706 and the support screen 704, permitting the lensdesigner to rely on a large refractive index difference when designingthe Fresnel structure. A larger refractive index difference permits thereduction in the angle of the functional portions of the Fresnelstructure 706, thus increasing the manufacturability of the Fresnellens. In addition, from a manufacturing viewpoint, there is a lowprobability of adhesive migrating into the grooves of the Fresnel lenswhen assembling the screen, and the use of truncated tips reduces theopportunity for damaging the tips of the Fresnel lens. Furthermore, theseparation between the ghost image and the primary image may besubstantially reduced, if not removed, because the Fresnel lens is inclose proximity to the support layer and the Fresnel lens may be made tobe thin, for example down to 0.010 inches or less.

Another advantage provided by the flat-top Fresnel lens 702 is that,even if not in direct contact with the second layer 704 a, the truncatedtips 708 and 709 permit the Fresnel structure to approach more to thesecond layer 704 a, for example as illustrated in FIG. 7B. The closeproximity between the lens 702 and the second layer 704 a, for example adispersing screen such as a diffuser or the like, reduces the separationbetween the ghost image and the primary image. In such a case, the lens702 may be held taut in a perimeter frame.

Another example of a screen that uses a flat-top Fresnel lens isillustrated in FIG. 7C. Here, the screen 750 includes a flat-top Fresnellens 752 attached to a beaded screen 754 having an upper transparentlayer 756, a layer of beads 758, and a lower transparent layer 760. Anopaque layer 762 prevents light from passing through the intersticesbetween the beads 758, and may also prevent reflection of ambient lightfrom the front of the screen 750. As is discussed in U.S. patentapplication Ser. No. 09/192,118, the gain of the screen may be adjustedby varying the difference in refractive index between the beads 758 andthe upper transparent layer 756. An advantage of using a flat-topFresnel lens 752 with the beaded screen 754 is that light passingthrough the Fresnel lens 752 passes into the air gaps 764 between theFresnel lens 752 and the upper transparent layer 756. Therefore, therefractive index of the upper transparent layer 756 may be adjusted toproduce a desired screen gain without affecting the re-directing, orcollimating, effect of the Fresnel lens 752.

The improvement in uniformity of brightness across a screen that may begained from using a flat-top Fresnel screen is illustrated in FIG. 8,which shows gain as a function of a viewing angle. Curves 802 and 804correspond to the gain curves for a diffusing screen alone, and werepreviously presented in FIG. 2. The upper curve 802 illustrates the gainfor light incident on the screen at an angle of θ=0°, while the lowercurve 804 illustrates the gain for light incident on the at the edge ofthe screen. For normal-incidence viewing at the center of the screen,the gain at the center (at a viewing angle of 0°) is 0.97, point D. Thegain at the edge of the screen (at a viewing angle of −30°) is 0.51,point E.

The brightness is significantly more uniform across the screen thatincludes the flattop Fresnel lens, curves 806 and 808. The gain measuredfor the center of the screen is represented by the solid curve 806 andthe gain measured for the edge of the screen is represented by thedashed curve 808. The solid curve 806 closely tracks the ideal curve fora Fresnel-collimated screen 802, except for a small loss in gain that iscaused by reflective loss introduced by the flat-top Fresnel lens. Thegain seen by a viewer at the center of the screen having the flat-topFresnel lens (viewing angle=0°) is 0.91, point F, while the gain at theedge of the screen (at a viewing angle of −30°) is 0.70, point G.Therefore, the drop in brightness from the center to the edge of thescreen with the flat-top Fresnel lens is approximately 23%. The drop inbrightness from center to edge for the ideal screen, curve 802, is about20%, very close to the value of 23% for the screen with the flat-topFresnel. Therefore, the flat-top Fresnel screen is effective atcollimating the light from the light source and making the screenbrightness uniform. The flat-top Fresnel lens has no additional supportother than the diffusing screen, and may be made sufficiently thin thatno ghost images are apparent to the viewer.

It should be noted that the Fresnel lens 542, illustrated in theembedded Fresnel embodiment of FIG. 5C, may be embedded into the centrallayer 546 to a depth where the optically inactive portions of theFresnel structure are embedded while the optically active portions 550of the functional slopes 552 are exposed to the air gaps. Such a designpermits the Fresnel lens 542 to operate with a large refractive indexdifference between the Fresnel lens and the air, while maintaining thesupporting function of the support layer, and also while holding thethin Fresnel lens close to the support layer to reduce the appearance ofghost images.

Another approach to supporting a Fresnel lens on a support layer isillustrated with respect to FIGS. 9-12. This approach is termed a“partial Fresnel”.

Screens having a Fresnel lens typically have the Fresnel structurecovering the entire screen. This is illustrated in FIG. 9, where thescreen 900 is formed from a Fresnel lens 902 and a support layer 904.The Fresnel structure 906 on the Fresnel lens 902 covers the entiresurface of the screen 900.

One method of fabricating a screen 900 is to apply the Fresnel lens 902to the support layer 904. However, certain difficulties may arise inapplying a Fresnel lens to a support layer 904. For example, theradially grooved structure 906 of the Fresnel lens 902 may lead to airentrapment when laminating one film to another, resulting in cosmeticdefects apparent to the viewer. Therefore, it may be advantageous toavoid having the Fresnel structure where it is not needed. Inparticular, from the discussion above with regard to FIG. 8, it isapparent that redirection is required mostly at the side edges,especially across the width of the screen, and at the four corners ofthe screen to increase perceived brightness in these areas, while littleredirection, if any, is required at the center of the screen.

Therefore, a screen may include a Fresnel lens that has a Fresnelstructure only at those portions of the screen where redirection isrequired, e.g. at the edges and corners. Such an approach is illustratedin FIG. 10, which shows a screen 1000 with a Fresnel pattern 1002 at theedges and corners of the screen 1000. The central portion 1004 of thescreen does not have any Fresnel structure, since there is less need forredirection of light in the central portion 1004 of the screen 1000. Anadvantage of this embodiment is that the number of defects arisingduring manufacture, e.g. due to air entrapment, may be reduced, thusenhancing the yield of acceptable screens. Additionally, since airfacets are eliminated in the central portion and the angle of incidenceis low, thus reducing reflection losses, the gain of the central portionof the Fresnel lens may be increased.

One particular approach to implementing a partial Fresnel lens isillustrated in FIG. 11A, which shows a cross-section of a screen thatuses a Fresnel film 1102 attached to a support layer 1104. The Fresnelfilm 1102 has a central portion 1106 that lacks a Fresnel structure andis, in this case, essentially flat. The edges of the Fresnel film 1102are provided with a Fresnel grooved structure 1108 to redirect the lightpassing through the edges 1114 of the screen 1100.

Here, the term attached is used to describe any method by which theFresnel lens is joined to the support layer, such as adhesion, with orwithout an adhesive layer, lamination, heat lamination, fusion, orultrasonic or RF bonding or welding, or the like.

In this particular embodiment, the Fresnel lens 1102 is also a flat-toplens where the tips of the Fresnel structure have flat surfaces 1110 forcontacting to the support layer 1104.

In another embodiment 1120, illustrated in FIG. 11B, the Fresnel lens1122 includes an output face 1124 having a flat central portion 1126.The Fresnel structure 1128 at the edges of the lens 1122 has tips 1130formed by the functional and riser slopes. The height of the flatcentral portion 1126 is selected to be approximately the same height asthe tips 1130 so that the tips 1130 are close to contacting, or are incontact with, the surface 1132 of the support layer 1104.

Another approach to a partial Fresnel structure, that also includes anembedded structure, is illustrated in FIG. 12. The screen 1200 has aFresnel lens 1202 embedded in a support layer 1204. The Fresnel lens1202 has a central region 1206 that is free of a Fresnel structure andis attached to the support layer 1204. There is a Fresnel structure 1208at the edges of the Fresnel lens 1202 that is embedded in the supportlayer 1204. The refractive index of the support layer 1204 is less thanthe refractive index of the Fresnel film 1202. The Fresnel structure1208 is designed to redirect light entering the support layer 1204 fromthe Fresnel film 1202.

It will be appreciated that other combinations of approaches may be usedfor manufacturing a screen with a supported, thin Fresnel lens. Forexample, a flat-top Fresnel lens may be embedded in a support layer,either having a full Fresnel pattern or a partial Fresnel pattern.Furthermore, a partial Fresnel lens may be partially embedded in asupport layer, or a central layer. These are only examples of otherapproaches, and are not intended to limit the invention.

Another approach to reducing the problem of trapping air bubbles whenthe Fresnel lens is laminated to a screen is illustrated in FIGS.15A-15D. In this approach, a Fresnel lens 1500 has a full Fresnelstructure 1502. Air-relief grooves 1504 cut across the Fresnel structure1502 permit the passage of air out of one valley of the Fresnelstructure into another valley when the lens is being laminated to itssupporting layer or when the Fresnel lens is being manufactured. Theair-relief grooves 1504 may be cut in different patterns across theFresnel structure 1502. FIG. 15A illustrates a partial radial pattern ofair-relief grooves 1504, such as may be used when the lamination processproceeds in a direction substantially parallel to the arrow. FIG. 15Billustrates a full radial pattern of air relief grooves 1504.

A partial linear pattern of air relief grooves 1504 is illustrated inFIG. 15C, and a full linear pattern of grooves 1504 is illustrated inFIG. 15D. Such linear patterns may be used when the lamination orFresnel lens manufacturing process proceeds in a direction illustratedby the respective arrows, i.e. substantially parallel to the grooves1504.

The air-relief grooves 1504 need not be straight, and may take on othershapes so long as they provide relief for air to flow from one valley inthe Fresnel structure to another valley.

Another embodiment of a Fresnel-screen that reduces the effect of ghostimages is illustrated in FIG. 16. The screen 1600 includes a frontsurface Fresnel lens 1602 attached, for example by lamination, to adiffusing layer 1604. The diffusing layer 1604 may be attached to asubstrate layer 1606 to provide support. An advantage of this screen1600 is that the “land” between the Fresnel surface 1608 and thediffusing layer 1604, in other words the thickness of Fresnel lens 1602between the surface of the diffusing layer 1604 and the bottom 1610 ofthe groove between the riser slope 1612 and the functional slope 1614,may be very small. This reduces the separation of the ghost image fromthe primary image.

A projection system that employs a screen with a thin Fresnel lens isillustrated in FIG. 13. The projection system 1300 includes a lightprojector 1302 that includes a light source 1304 (l.s.) that generates abeam of light 1305. The beam of light 1305 may propagate through beamhandling optics 1306 before illuminating a reflective polarizer 1308,for example a reflective polarizing sheet as described in PCTpublication WO 96/19347. Light of a certain polarization is reflected bythe polarizing beam splitter 1308 to a LCD array 1310. The LCD arrayreflects the light back towards the polarizing beam splitter 1308. TheLCD array 1310 spatially modulates the light beam 1309 incident thereonby rotating the polarization through approximately 90°. Therefore, thoseportions of the light beam 1311 reflected by the LCD array 1310 whosepolarization is rotated by the array 1310 are transmitted by thepolarizer 1308 as beam 1313. The beam 1313 may pass through transmissionoptics 1312 before illuminating the screen 1314. The transmission optics1312 may include, for example, projection lenses and/or a polarizer forcleaning up the image on the screen 1314. The screen 1314 includes aFresnel lens 1316 followed by a diffusing screen 1318. As discussedabove, the diffusing screen may be a bulk diffuser, a surface diffuser,a beaded screen, or the like. The screen 1314 may be any one of theembodiments described above, or a combination thereof, in which a thinFresnel lens 1316 is supported on a diffusing screen 1318. Light fromthe screen 1314 is detected by the viewer at position 1320.

FIG. 13 does not show the lateral extent of any of the light beams, butindicates a central ray in each light beam. The lateral extent of thebeams is determined, at least in part, by the particular beam handlingoptics 1306 and transmission optics 1312 employed in the projectionsystem 1300. The projection system 1300 may include one or more foldingmirrors to reduce the depth of the system. When the form factor of thesystem is made smaller for a given size of screen, the divergence oflight along the light path between the light source and the screenincreases. This typically increases the need for a redirecting lens,such as a Fresnel lens, at the screen to maintain brightness uniformityand efficient light use.

It will be appreciated that the projection system need not be configuredexactly as shown. For example, transmission optics may be positionedbetween the polarizer 1308 and the LCD array 1310 in addition to, orinstead of, the transmission optics 1312 between the polarizer 1308 andthe screen 1314. In addition, the projection system may be configuredusing a transmissive LCD display, rather than a reflective LCD display.

It will be appreciated that the embodiments presented above have beenused for illustrative purposes, and that certain features of theillustrated embodiments may be changed without affecting the presentinvention. For example, the Fresnel lens need not have a circularFresnel pattern, as illustrated, but may have a Fresnel pattern which isa linear Fresnel pattern, for redirecting light along one axis, or mayalso be a two-dimensional Fresnel pattern, other than circular, forredirecting light along two axes.

As noted above, the present invention is applicable to display systemsincorporating a Fresnel lens. It is believed to be particularly usefulin reducing the effect of ghost images in back projection displays andscreens. The use of the Fresnel lens of the present invention permitsreduction in the form factor of the screen and high light useefficiency, while reducing ghost images to permit high resolutionoperation. Accordingly, the present invention should not be consideredlimited to the particular examples described above, but rather should beunderstood to cover all aspects of the invention as fairly set out inthe attached claims. Various modifications, equivalent processes, aswell as numerous structures to which the present invention may beapplicable will be readily apparent to those of skill in the art towhich the present invention is directed upon review of the presentspecification. The claims are intended to cover such modifications anddevices.

We claim:
 1. A Fresnel lens comprising an optically transmitting layerhaving an input surface and an output surface, at least a portion of theoutput surface including a Fresnel structure; and optically absorbingparticles disposed within the optically transmitting layer, wherein atleast a portion of the output surface lacks the Fresnel structure.
 2. AFresnel lens as recited in claim 1, wherein the portion of the outputsurface lacking the Fresnel structure is disposed towards a centerportion of the optically transmitting layer.
 3. A Fresnel lenscomprising an optically transmitting layer having an input surface andan output surface, at least a portion of the output surface including aFresnel structure; and optically absorbing particles disposed within theoptically transmitting layer, further comprising air-relief groovesdisposed across the output surface.
 4. A Fresnel lens comprising anoptically transmitting layer having an input surface and an outputsurface, at least a portion of the output surface including a Fresnelstructure; and optically absorbing particles disposed within theoptically transmitting layer, wherein the Fresnel structure includesridges formed between respective riser slopes and active slopes, theridges having flat portions parallel to the optically transmittinglayer.
 5. A Fresnel lens as recited in claim 4 wherein the flat portionof at least one of the ridges extends beyond the plane of the respectiveactive slope.
 6. A Fresnel lens comprising an optically transmittinglayer having an input surface and an output surface, at least a portionof the output surface including a Fresnel structure; and opticallyabsorbing particles disposed within the optically transmitting layer,further comprising a supporting layer supportingly attached to theoptically transmitting layer.
 7. A Fresnel lens as recited in claim 6,wherein at least a portion of the Fresnel structure is embedded in thesupporting layer.
 8. A Fresnel lens as recited in claim 6, wherein theFresnel structure includes ridges having fiat portions parallel to thesupporting layer, the flat portions being attached to the supportinglayer.
 9. A Fresnel lens, comprising: an optically transmitting layer,having an input surface and an output surface, the output surfaceincluding a plurality of ridges formed by respective riser slopes andactive slopes to form a Fresnel structure, one of the active slopesdefining a plane and a tip end of at least one of the ridges having aportion that extends beyond the plane of the active slope.
 10. A Fresnellens as recited in claim 9, wherein at least a portion of the outputsurface lacks the Fresnel structure.
 11. A Fresnel lens as recited inclaim 10, wherein the portion of the output surface lacking the Fresnelstructure is disposed towards a center portion of the opticallytransmitting layer.
 12. A Fresnel lens as recited in claim 9, furthercomprising air-relief grooves disposed across the output surface.
 13. AFresnel lens as recited in claim 9, further comprising optically activeparticles disposed within the optically transmitting layer.
 14. AFresnel lens as recited in claim 13, wherein the optically activeparticles are light diffusing particles.
 15. A Fresnel lens as recitedin claim 13, wherein the optically active particles are light absorbingparticles.
 16. A Fresnel lens as recited in claim 9, further comprisinga supporting layer supportingly attached to the optically transmittinglayer.
 17. A Fresnel lens as recited in claim 16, wherein at least aportion of the Fresnel structure is embedded in the supporting layer.18. A Fresnel lens as recited in claim 16, wherein the Fresnel structureincludes ridges having flat portions parallel to the supporting layer,the flat portions being attached to the supporting layer.
 19. A Fresnellens as recited in claim 16, wherein the portion of the tip endextending beyond the plane of the active slope includes a flat portionparallel to the supporting layer, the flat portion attached to thesupporting layer.